236 INFECTION AND RESISTANCE 



altered agglutinins. In support of this assumption it has been 

 further shown that sera which 'have been rendered inhibitory by 

 either of the methods named can be deprived of their inhibiting 

 characteristics by absorption with homologous bacteria. Together 

 these observations constitute a strong argument in favor of the ag- 

 glutinoid theory. 



In practical experience the existence of such an inhibition zone 

 is of great importance, since freshly taken sera will occasionally 

 show this failure of agglutination in concentration, while strong 

 agglutination follows when the dilution is increased. In clinical 

 tests, as in the Widal reaction for the diagnosis of typhoid fever, 

 we not infrequently encounter sera which will give no agglutination 

 in dilutions of 1 to 20 and even 1 to 40, and the reaction would 

 therefore be regarded as negative unless the possibility of the pro- 

 agglutinoid zone were recognized and further dilutions carried out. 



While there is no question of the accuracy of the experimental 

 data cited in the preceding paragraphs, the interpretation of the phe- 

 nomena on the basis of Ehrlich's haptine conception has not been 

 universally accepted. 



The fact that the agglutinins lose their agglutinating power after 

 heating, while retaining their power to prevent the subsequent agglu- 

 tination of the bacteria, may be more simply explained in analogy 

 with the observation of Forges on the influence of heated serum upon 

 the agglutination of mastic suspensions. He found that unheated 

 serum will flocculate such suspensions, while heated serum of the 

 same concentration will prevent the flocculation, acting probably as 

 a protective colloid (see chapter on Colloids). In the same way the 

 heated agglutinating serum may prevent subsequent flocculation by a 

 similar protective action. We suggest this as a possible explanation 

 of the proagglutinoid phenomenon, although of course it is a mere 

 conjecture as opposed to the painstaking experiments of Eisenberg 

 and Volk. It has the advantages of simplicity, but does not, it is 

 true, explain the apparent specific absorption of the agglutinin- 

 inhibiting properties out of heated sera with homologous bacteria, as 

 claimed by these authors as well as Kraus and Joachim. The simi- 

 larity of the proagglutinoid phenomenon to the inhibition zones oc- 

 curring in the flocculation of colloids will be referred to in a subse- 

 quent paragraph. 



In describing the investigations which led to the discovery of 

 the mechanism of "the lytic phenomenon, in the chapter on Cytolysis, 

 we mentioned that Bordet and others had noticed the frequent ag- 

 glutination of red blood cells, in the sera of animals treated with 

 such cells after the hemolytic property had been destroyed by heat- 

 ing to 56 C. Such hemagglutination is a phenomenon entirely 

 analogous to the agglutination of bacteria by serum, and hemag- 

 glutinins regularly result when an animal is systematically treated 



